Migration guide promises the Nuke.* shim is removed in 12.0

### Problem

`docs/migration/from-nuke.md:102` tells consumers, as a caveat on the transition-shim path:

> The shim is a transition aid, not a permanent home. It's removed in 12.0. Plan to migrate before then.

There is no 12.0 and there will not be one. The `11.x` line was skipped and the project moved from `10.x` straight to calendar versioning ([ADR-0004](docs/adr/0004-calendar-versioning-and-dual-pace-channels.md)); the stray `11.0.x` packages on nuget.org are unlisted. So the single deadline consumers are given for the shim's removal names a version that will never ship — in the page the `FALLOUT004` diagnostic's help link points at, which is the page someone reads precisely when deciding whether the shim is safe to sit on.

Found while removing `docs/rebrand-plan.md` in #591. The same claim was in `src/Fallout.Migrate.Analyzers/README.md` ("ship through the 11.x line, remove in 12.0") and is fixed there; #591 was kept scoped to the doc removal, so the consumer-facing copy is still live.

### Outcome

The migration guide states the shim's removal in terms that stay true as the version line moves: it is a breaking change, so it lands at a yearly major cut, and it is undated until that cut is chosen. A consumer reading it can tell whether they have to act now, without being given a fake deadline.

### Acceptance criteria

- [ ] `docs/migration/from-nuke.md` no longer references `12.0`.
- [ ] The stated policy matches the actual release model — shim removal is breaking, therefore batched to a yearly major (AGENTS.md rule #1, ADR-0004) — rather than naming a version.
- [ ] The caveat still tells the reader the shim is transitional and that migrating is the durable option; softening the deadline should not read as "the shim is permanent".

### Notes

Related but out of scope: whether the shim moves off GitHub Packages to drop the PAT requirement (#344), and the incomplete shim surface (#69) — both already have their own caveats on that page.

Open docs/migration/from-nuke.md around line 102, then read AGENTS.md rule #1 and docs/adr/0004-calendar-versioning-and-dual-pace-channels.md to confirm the release policy. Update the caveat so it describes removal at a chosen yearly major cut without naming 12.0, while retaining the transitional warning and durable-migration guidance.

Written by the indexing model from the issue text.